The present invention relates to a high-pressure fuel pump drive circuit which is designed to control electric current on the occasion of driving a high-pressure fuel pump for engine so as to decrease the fall time of electric current flowing into the load having inductance.
Prior arts to the present invention are disclosed, for example, in JP Published Patent Application 2002-237412 A, JP Published Patent Application H8-55720 A and Watanabe “Practical Method for the Design of Analog Electronic Circuit” Sogo denshi Press 1996.
FIG. 1 illustrates a conventional circuit configuration of a high-pressure fuel pump drive circuit for engine. In this circuit, the solenoid coil 2 of high-pressure fuel pump is connected with the drain of switching MOSFET (Nch) 3 and furthermore, the cathode of a flywheel diode 1 is connected with a source voltage VB and the anode of the flywheel diode 1 is connected with the solenoid coil 2. When an input voltage is applied to the gate of MOSFET (Nch) 3, the MOSFET (Nch) 3 is turned ON, permitting an electric current IL to pass to the solenoid coil 2. At this moment, the drain voltage VD of MOSFET (Nch) 3 is caused to drop from VB to about 0 volt and, at the same time, the electric current IL passing through the solenoid coil 2 is caused to rise transiently and electromagnetic energy is caused to accumulate in the solenoid coil 2 due to this electric current IL.
When the input voltage to the gate of MOSFET (Nch) 3 is dropped to 0 volt, a power to force electric current to flow in the direction to inhibit any changes of magnetic flux is acted thereon due to the self-induction electromotive force (e=L*ΔI/Δt) by the electromagnetic energy. As a result, the electric potential of VD is caused to rise, whereby large voltages, opposite in direction, are imposed on the opposite ends of the solenoid coil 2, respectively. These large voltages developed on the opposite ends of the solenoid coil 2 can be vanished by passing electric current to the flywheel diode 1 which is connected, in parallel, with the solenoid coil 2.
Meanwhile, in a steady state wherein the MOSFET (Nch) 3 is turned ON and an input voltage as indicated by the number 5 in FIG. 2 is given thereto, since the time for shifting the MOSFET (Nch) 3 from OFF to ON can be made shorter as the switching cycle is made faster, the magnitude of voltage to be developed at the opposite ends of solenoid coil 2 can be confined to a small value and, at the same time, the magnitude of energy to be consumed by the flywheel diode 1 can be minimized, thereby making it possible to minimize the generation of heat in the device.
Whereas, when the MOSFET (Nch) 3 is kept in a state of OFF for a relatively long time as indicated by the number 6 in FIG. 2, the electric current to be fed to the solenoid coil 2 having inductance would become zero, thereby permitting an induced electromotive force to generate due to the decrease of the magnetic flux of solenoid coil 2. As a result, an electric current ID is permitted to pass through the flywheel diode 1. In conformity with the decrease of the induced electromotive force, this electric current ID becomes zero after a predetermined period of time though it is accompanied with a relatively long time constant. Namely, the fall time of this electric current ID to be passed to the solenoid coil 2 would be prolonged. As long as this condition is kept unchanged, the controllability of high-pressure fuel pump would be deteriorated and hence the fuel pressure cannot be stabilized. Further, when the rotational speed of engine is increased, there are many possibilities that unintentional behavior of fuel pressure may be caused to occur. Therefore, it may be required to employ a Zener diode in order to shorten the fall time of electric current.
FIG. 3 illustrates another conventional circuit configuration wherein a Zener diode is additionally provided. This circuit configuration differs from that of FIG. 1 in the respects that the cathode of Zener diode 8 is connected with the solenoid coil 7 and the anode of Zener diode 8 is connected with the ground GND, and, additionally, the switching MOSFET (Nch) 9 is connected, in parallel, with the Zener diode 8, thus omitting the flywheel diode. Because, if the flywheel diode is kept unremoved, it would make the Zener diode quite inoperative, thereby rendering the circuit configuration of FIG. 3 the same in function as that of the conventional circuit configuration shown in FIG. 1.
When the switching of steady sate wherein an input voltage as indicated by the number 5 in FIG. 2 is impressed is applied to the MOSFET (Nch) 9, the electric current would be clamped by the Zener diode 8 every occasion the MOSFET (Nch) 9 is turned OFF, thereby rendering the Zener diode 8 to generate such a large magnitude of heat that the device can no longer withstand the heat thus generated.
Therefore, it is required to shorten the fall time of electric current flowing into the solenoid coil and also to suppress the generation of heat from the device.